1. Field of the Invention
The invention relates to arrays of fiber optic sensors, particularly with respect to multiplexing arrangements therefor. The invention is specifically applicable to multiplexing arrays of variable coupler fiber optic sensors.
2. Description of the Prior Art
The variable coupler fiber optic sensor is adaptable for a variety of parameters such as temperature, pressure, sound and the like. The sensor is described in U.S. Pat. No. 4,634,858, issued Jan. 6, 1987, entitled "Variable Coupler Fiber Optic Sensor" and assigned to the assignee of the present invention. Said U.S. Pat. No. 4,634,858 is incorporated herein by reference. Applications of the sensor are disclosed in U.S. patent application Ser. No. 376,342, filed July 6, 1989, entitled "Variable Coupler Fiber Optic Sensor Hydrophone", by David W. Gerdt; and Ser. No. 444,920, filed Dec. 4, 1989, entitled "Method of Monitoring Cardiovascular Signals and Fiber Optic Coupler Phonocardio Sensor Therefor", by David W. Gerdt. Said Ser. Nos. 376,342 and 444,920 are assigned to the assignee of the present invention and are incorporated herein by reference.
Briefly, as described in said U.S. Pat. No. 4,634,858, the sensor comprises a plurality of input optical fibers, each having a core, the cores of the optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge. Light energy from, for example, a laser or light emitting diode incident to one of the input fibers is distributed to the plurality of output fibers. The waist region is encapsulated in material having a refractive index variable with stress applied thereto and the applied stress varies the distribution of output light energy. A differential detector such as a plurality of photodiodes is coupled to the output fibers for providing signals representative of the optical energy distribution in the output fibers. The sensor may be constructed in accordance with teachings in U.S. patent application Ser. No. 240,986, filed Sept. 6, 1988, entitled "Fiber Optic Fabrication Furnace", by David W. Gerdt. Said U.S. patent application Ser. No. 240,986 is assigned to the assignee of the present invention and is incorporated herein by reference.
Thus, a sensor includes at least one input fiber requiring a light source and generally two output fibers coupled to a detector for providing a signal related to the physical parameter being sensed.
It is often desirable to configure such sensors into arrays such as linear arrays or nets of plural sensors, each requiring a light source and output detectors. For example, an array of hyrophone sensors may be mounted on the outer hull of a marine vessel such as a submarine or an array of pressure sensors may be utilized to detect levels of liquids such as in oil tankers. In such arrays, it is desirable to minimize the number of input and output leads so as to minimize hull penetration fittings. Independent sensors for constructing such arrays, each sensor requiring a separate light source and detector, results in excessive complexity and cost.
It is a desideratum to reduce the large number of input and output fibers from individual sensors utilized in such arrays. A variety of techniques are known in the art for multiplexing a plurality of fiber optic sensors of types other than the variable coupler fiber optic sensor, such as the interferometric type. The prior art does not provide a useful multiplexing configuration for the variable coupler fiber optic sensor. It is believed that the prior art does not provide a useful multiplexing configuration for fiber optic sensors of the intensity type such as the micro-bending sensor.
Configurations for multiplexing fiber optic sensors of the interferometric type are discussed in "Fiber-Optic Multisensor Networks", SPIE Proceedings, Volume 985, (1988) by Kersey and Dandridge. Such multiplexing configurations are undesirably complex since interferometric sensors require preservation of precise phase information. A reference leg and a sensor leg are always required for Michelson and Mach-Zehnder sensors. Additionally, interferometric sensors are difficult to deploy and are extremely expensive.